A tomography appliance with an X-ray emitter and a detector is known, for example, from DE 195 02 574 C2. The detector includes a plurality of detector elements, which are arranged in a rectangular detector array formed from rows and columns. The X-ray emitter may in this case be an X-ray tube. However, any other desired sources of X-ray radiation are also feasible.
The detector elements are used for production of detector output signals as a measure of the absorption of X-ray radiation which originates from the X-ray emitter and passes through a measurement area. The detector and X-ray emitter are arranged such that they can be rotated about a rotation axis. It is thus possible to reconstruct a three-dimensional image, for example in order to examine the interior of the body of a patient, for an object positioned in the measurement area, on the basis of the detector output signals obtained from different rotation angle positions.
By way of example, scintillation detectors or semiconductor detectors may be used in a tomography appliance. The detector elements in a scintillation detector as known from DE 100 51 162 A1 each have a scintillator and a photodiode associated with it. In a detector such as this, the detector output signals are produced indirectly by means of light pulses, which are caused by absorption of X-ray quanta in the scintillator.
The detector elements of a semiconductor detector that is known from U.S. Pat. No. 5,77,338 in contrast to this each have a p-doped and n-doped semiconductor material with a depletion layer which is sensitive to X-ray radiation. The detector output signals from a semiconductor detector such as this are produced directly from the charge carriers caused by an X-ray quantum in the depletion layer.
Scintillation detectors are preferably operated as integrating detectors. In this operating mode, the detector output signal is integrated over a specific time. A good image quality can be achieved, particularly if the detector elements have a short decay time.
Semiconductor detectors are, in contrast to this, preferably operated as counting detectors. After the occurrence of an event, a counting detector requires a specific time, the so-called dead time, to process this event. All other events which occur during this time are lost. A distinction is drawn between two situations in the behavior of a detector in the counting operating mode:
(1) non-paralyzing:
After each verified event, the detector is not sensitive for a fixed time τ, where τ corresponds to the dead time. It cannot register events which occur during this time (non-extendable dead time).
(2) paralyzing:
The detector remains sensitive even during the dead time. The dead time can thus be lengthened by the occurrence of a further event (extendable dead time).
When a large number of X-ray quanta arrive, there is a non-linear relationship between the measured counting rate and the counting rate which actually acts on a detector element.
The relationship between the measured and actual counting rate can be specified for detectors with a non-paralyzing behavior by m=n/(1+n*τ), and for detectors with a paralyzing behavior by m=n*e−nτ where m is the measured counting rate, n is the actual counting rate, and τ is the dead time of the detector elements.
The measured counting rate can also be understood as the measured intensity of the X-ray radiation, and the additional counting rate can also be understood as the actual intensity of the X-ray radiation acting on a detector element. Thus, in order to simplify the description, the following text also uses the generally descriptive expression intensity rather than counting rate.
An error in the determined intensity of the X-ray radiation for the respective detector element at the time of an examination or at the time of a calibration of the tomography appliance leads to a deterioration in the achievable image quality in the reconstruction of a three-dimensional image on the basis of different projection images which have been recorded at different rotation angle positions.